The present invention relates to a variable speed controlled induction motor in which the torque characteristics and efficiency are good and control of its speed is easy.
One of the conventional methods for controlling the speed of an induction motor is to vary the power supply frequency. Although this method enables the speed to be controlled continuously over a wide range, it requires a high cost frequency converter. Further, such a frequency converter, in the course of converting the AC current to DC current and back to AC current, may produce high harmonic noise and other electromagnetic waves which, if they flow into the commercial power lines, would cause various high harmonic noise problems, and malfunctioning of computers or other electronic devices, heating of capacitors, etc. As for the high harmonic noise problems, they may be eliminated if an appropriate high frequency filter is provided, but this also is expensive. Also, this method has drawbacks in that, in general, the motor does not operate efficiently at lower speeds.
A method by which the number of poles of the induction motor is varied in operation does enable the speed to be controlled, but only step-wise, and thus the control is neither continuous nor smooth.
A method by which the supply voltage is varied enables the speed to be controlled continuously but there is a drawback in that the motor does not operate efficiently at low speed ranges.
A method by which, in a wound rotor type motor, the secondary insertion resistance is varied, to vary the slip enables the speed to be continuously controlled comparatively easily but, since a resistance is inserted in the rotor winding circuits from the outside through brushes and slip rings, there is a drawback in that it is necessary to make periodic checks and perform careful maintenance of these brushes and slip rings. In induction motors of the squirrel-cage rotor type, a problem is that it is not possible to vary the second resistance in order to effect wherein the second resistance is varied in order to effect the control of speed.
Prior art which attempted to solve the above problems in available. For example, Unexamined Japanese patent publication No. 29005/1979 discloses an arrangement in which squirrel-cage conductors are disposed so that they extend along a pair of rotor cores, and both ends of these conductors are respectively short-circuited by means of short-circuit rings and in which a high resistor is disposed in such a way that the squirrel-cage conductors are short-circuited at a middle region of the conductors between the two rotor cores. Windings are provided on stators which are independent the corresponding rotor cores. In this squirrel-cage rotor induction motor, the phases are displaced by 180.degree. between the windings of the stators at the start of rotation, but when the motor is in operation after starting, the phases are reversed to be in phase with each other. Because of the phase displacement by 180.degree. between the windings of the stators upon starting, the starting torque increases so as to enhance the starting characteristics. However, during normal operation, as the phases of the stator windings are the same, it operates under normal torque characteristics. Therefore, although the starting characteristics may be recognized as improved, this motor is not a controllable speed motor and cannot be used as a drive motor for a load which requires variation in speed.
Unexamined Japanese patent publication No. 29005/1979 includes an example in which, in order to soften the shock which may otherwise be caused by the sudden change in torque in transition from the starting to the normal operation of the motor, the connection of the respective windings of the stators to the power supply circuit is momentarily put in series as an intermediate step to the full operation of the motor. This arrangement is limited to the displacement to either 0.degree. or 180.degree. of the phase of the rotating magnetic fields and is in no way intended to be used for varying speed. When the connection is changed to a series connection, the voltage applied to the stators is reduced by half, which means that the torque is reduced to 1/4. It is clear from this that with the arrangement disclosed in this published application it is impossible to effect the control of variations of speed.
Thus, although Unexamined Japanese patent publication No. 29005/1979 includes reference to series connection by stating that there ". . . may be an intermediate step of switching the windings of the stators between the series connection and the parallel connection with respect to the power supply circuit", it must be considered that this series connection has nothing to do with, and does not do anything for the purpose of speed control.
Unexamined Japanese patent publication No. 86807/1974 discloses a non-synchronous motor having a squirrel-cage rotor and a stator of multi-phase windings. It includes conductive bars, short-circuit end rings and ferro-magnetic layers. The stator is divided into first windings and second windings. These first and second windings are disposed coaxially adjacent to each other and adjacent to different portions of the rotor. AC current of the same frequency may be supplied. There is also provided a means to vary the power induced in the windings of the rotor by the second windings of the stator. In this motor, either by mechanical means or electrical means, it is possible to establish a phase difference between the two divided portions of the stator and to change to some extent the rotational speed of the rotor. However, except during the period in which the phase angle between the two divided stator portions is in the same phase, the torque is so small that the motor immediately stops when the load thereto becomes large and this makes the motor unsuited for practical use. Thus, this motor cannot be used in a situation where the motor needs to be repeatedly and frequently started and stopped under the state in which the motor is kept loaded, which means that this motor has not solved the abovementioned problems.
The present invention aims at overcoming the problems seen in the prior art and achieving advantages which cannot be obtained even by the combination of the above explained Unexamined Japanese patent publication No. 29005/1979 and Unexamined Japanese patent publication No. 86807/1974, such as the achieving of special torque characteristics, the possibility of setting any desired speed in a wide range of smoothly controlled speeds, and the possibility of starting the motor at any desired torque. In the variable speed induction motor of the present invention, the torque characteristics are excellent and have high efficiency. These are achieved due to the application of a rating current approximately the same as that applied during the full rotation speed operation of the motor over a wide speed range from starting to the full rotation speed.
The variable speed controlled induction motor of the present invention may be used with either a single-phase power source or three-phase power source. The form of the rotor may normally be of any type such as squirrel-cage, a double squirrel-cage, a deep-slot squirrel cage, a special squirrel-cage or a wound type. The conductors or conductive members used in the description of the present invention generally refer to those provided in the rotor cores of the squirrel-cage type and windings wound on the rotor cores of the wound type.
According to the present invention there is provided a variable speed controlled induction motor comprising: a rotor formed in one piece having a plurality of rotor cores mounted, with a predetermined space being provided therebetween, on a common axis and having a plurality of conductive members interconnected and respectively mounted on the rotor cores; a plurality of stators disposed side by side and surroundingly facing the respective rotor cores and having stator windings wound respectively on the stators and connected in series; connecting members short-circuiting the conductive members at an air space or non-magnetic core portion disposed at the space between the rotor cores; and phase shifting means for producing phase differences between the voltages induced on the portions of the conductive members which face one of the plurality of stators and the voltages induced on the corresponding portions of the conductive members which face another one of the stators.
According to the present invention, it is possible to change the rotational speed of the rotor by means which enables creation of a displacement in the flux of the magnetic fields generated between the respective stators and the rotor, and by means by which the voltage induced in the rotor conductive members is controlled so as to be increased or decreased according to the displacement in the phase of the magnetic fields.
The arrangement wherein the respective windings wound on a plurality of stators are connected in series and the arrangement wherein a plurality of the conductive members mounted on a plurality of rotor cores are short-circuited through respective connecting members to produce the following two functions. One function is that the amount of current which flows in the rotor conductive members based on the voltages induced at those portions of the rotor conductive members which correspond to the respective stators becomes the same, and the other function is that the current corresponding to the amount of the vector difference caused by the phase difference of the voltages between the stators flows through the connecting members, short-circuiting a plurality of the rotor conductive members. By the synergistic effects of these two functions, the extent to which the resistance has an effect on the current flows, based on the phase difference .theta. of the voltages, can be completely controlled. In other words, by varying the phase difference .theta., the ratio between the current which flows in the rotor conductive members and the current which flows in the connecting members can be controlled independently of the slip. As a result of this, when the amount of the phase difference .theta. is controlled up and down, the function of the connecting members connecting the rotor conductive members is controlled in a desired amount and in a stable manner so that it is possible to obtain a large stable torque, not only at the starting of the motor, but also through a wide range from low speed operation, to high speed operation, thereby improving the overall efficiency.